The graphical plan provides more information than the short or long text plans. The graphical plan with statistics, though more expensive to generate, provides the actual query execution statistics as monitored by the database server when the query is executed, and permits direct comparison between the estimates used by the optimizer in constructing the access plan with the actual statistics monitored during execution. Significant differences between actual and estimated statistics might indicate that the optimizer does not have enough information to correctly estimate the query's cost, which may result an inefficient execution plan.
To generate a graphical plan with statistics, the database server must execute the statement. The generation of a graphical plan for long-running statements might take a significant amount of time. If the statement is an UPDATE, INSERT, or DELETE, only the read-only portion of the statement is executed; table modifications are not performed. However, if a statement contains user-defined functions, they are executed as part of the query. If the user-defined functions have side effects (for example, modifying rows, creating tables, sending messages to the console, and so on), these changes are made when getting the graphical plan with statistics. In some cases you can undo these side effects by issuing a ROLLBACK statement after getting the graphical plan with statistics. See ROLLBACK statement.
|Envoyer votre avis sur cette page par email.||Copyright © 2009, iAnywhere Solutions, Inc. - SQL Anywhere 11.0.1|